In order to facilitate an understanding of the present invention, a description is first given of the current state of the conventional art, with reference to FIG. 7 and FIG. 8.
FIG. 7 is a diagram showing the general structure of a vacuum surface hydrostatic bearing using a differential exhaust apparatus installed inside a vacuum chamber. FIG. 8 is a diagram showing the differential exhaust apparatus (frame) shown in FIG. 7 viewed from a guide surface.
As shown in the diagrams, a hydrostatic pad 5 is mounted on a frame 4, with a labyrinth seal surface and vacuum pockets 61-63 disposed so as to surround the hydrostatic pad 5. The labyrinth seal parts of a guide 9 and the frame 4 are disposed opposite each other across a slight gap. A gas is supplied to the hydrostatic pad 5 at #4 typically at a pressure of 1 atmosphere or more via an intake tube 8, enabling the hydrostatic pad 5 to act as a hydrostatic bearing. The gas so supplied, which may be He, N2, air or some other suitable gas or gas mixture, passes through the slight gap between the labyrinth seal and the guide 9 and flows into a first vacuum pocket 61 and is expelled by a vacuum pump 21 via an exhaust tube 71. When the conductance at the labyrinth seal is very small the first vacuum pocket 61 is evacuated, so a large pressure differential between the pressure #4 of the bearing assembly (that is, the pressure of the hydrostatic pad 5 assembly) and the pressure #3 of the first vacuum pocket 61 can be generated.
In addition, gas that has not been fully exhausted via the exhaust tube 71 passes further through the slight gap between the labyrinth seal and the guide 9 and flows into a second vacuum pocket 62, where the gas is then exhausted by a second vacuum pump 22. By so doing, the pressure #2 at the second vacuum pocket 62 is reduced below the pressure #3 of the first vacuum pocket 61.
By similarly further reducing the pressure toward the periphery so that the pressure (#1) of an outermost vacuum pocket 63 approaches the pressure (#0) of the interior of a chamber 10, the leakage of gas from the hydrostatic pad 5 to the chamber 10 can be minimized. That is, the low-conductance effect at the labyrinth seal part and the exhaust effect of the vacuum pockets 61-63 are disposed so that the relative pressures of the constituent parts are #4>#3>#2>#1>#0, where the typical pressures #1 through #3 are much less than 10,000 Pa.
In addition, outside the chamber 10 the exhaust tubes 71-73 are connected to the vacuum pumps 21-23 either directly or via valves 11a-13a, so as to evacuate the vacuum pockets 61-63.
In the differential exhaust mechanism described above, the hydrostatic pad 5 and its associated members are designed to form a stable bearing where #0≦#1≦#2≦#3≦#4 and where the difference between pressures #1-#3 and #0 is much less than 10,000 Pa. That is, the design is such that, under the foregoing conditions, a force in a direction pulling away from the guide surface 9 caused by the load rigidity of the hydrostatic pad 5 and the force of gravity and the like pressing the frame against the surface of the guide 9 are evenly balanced. The pressures #3-#1 exerted on the vacuum pockets 61-63 are virtually identical to the chamber pressure #0, and do not contribute to the force pulling away from the surface of the guide 9.
However, when the difference in pressure between #1-#3 and #0 is 10,000 Pa or greater, the vacuum pocket parts 61-63 also function as part of the bearing, with possible loss of bearing stability and oscillation. Similarly, because bearing stability deteriorates sharply when the hydrostatic pad 5 is not supplied with gas and the vacuum pockets 61-63 function as bearings, there is an additional risk that at even lower pressure differentials the frame 4 will oscillate vertically.
In order to prevent such oscillation, it is necessary to control the pressure of the vacuum pockets. However, coupling the exhaust directly to pumps 21-23 as in the conventional art makes it highly probably that the aforementioned pressure state will not be maintained when the pumps malfunction or when the apparatus is being started and the vacuum for the chamber 10 is being formed.
For example, when the pressure #0 inside the chamber is 1 Pa or less and the conditions described above are established and any one of the differential exhaust vacuum pumps 21-23 malfunctions, so that any one of pressures #1 through #3 approaches atmospheric pressure, the pressure differential with the pressure #0 inside the chamber increases to 10,000 Pa or more (that is, #0<<#1, #2, #3) and a bearing effect of the vacuum pockets 61-63 not considered at the design stage is generated, which can lead to a breakdown in bearing balance and consequent oscillation. In such a case, even if the malfunction of the vacuum pumps 21-23 is detected and the corresponding valves 11a through 13a are closed quickly, when oscillation begins it will not cease unless the pressure of the vacuum pockets 61-63 is reduced. Oscillation can occur easily, depending on the characteristics of the apparatus, during start-up and shut-down of the apparatus, that is, when gas is not being supplied to the hydrostatic pad 5.
Yet, given the conventional structure, it is difficult to maintain design pressures #0 through #4 when the apparatus is being started and the vacuum chamber 10 is being reduced to a vacuum from atmospheric pressure. That is, even if valves 11a-14a shown in FIG. 7 are connected and vacuum pumps 21-24 are all activated simultaneously, due to such factors as differences in exhaust capacity, the pressures #1-#3 inside the vacuum pockets 61-63 drop more rapidly than the pressure #0 inside the chamber such that #0>>#1, #2, #3 and the pressure differential exceeds 10,000 Pa. In such a case, in the vacuum pockets 61-63 parts, a force occurs that normally does not arise and that presses against the guide becomes excessive, posing a risk of deforming the structure that supports the frame 4. Likewise, evening when the valves 11a-13a connected to the differential exhaust are shut and only the chamber 10 is evacuated, because the gas can only move through the slight gap I the labyrinth seal a pressure differential of 10,000 Pa or more still occurs between the interior of the chamber 10 and the vacuum pockets 61-63 so that #0<<#1, #2, #3 and a force pulling away from the guide 9 begins to act, unbalancing the bearing and inviting oscillation. Usually the vacuum is formed without supplying gas to the hydrostatic pad 5, so the slight gap in the labyrinth seal becomes smaller still, inviting pressure differentials and further destabilizing the bearing so as to invite oscillation.
In other words, the problem with the conventional art is that, although under normal conditions there is no difficulty, when forming a vacuum in the chamber or when the exhaust system malfunctions (that is, the pump breaks down or the tubing fractures) there is a possibility that the pressure differential between the vacuum pockets 61-63 and the interior of the chamber 10 may exceed permissible design limits, yet the conventional art offers no structure to respond to such a situation.